Problem: Which of these numbers is composite? ${3,\ 53,\ 61,\ 79,\ 99}$
Solution: A composite number is a number that has more than two factors (including 1 and itself). 3, 53, 61, and 79 each have only two factors. The factors of 99 are 1, 3, 9, 11, 33, and 99. Thus, 99 is the composite number.